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Spot film camera (Photospot) (105 mm in this case) Cine (35 mm) camera Video camera Spot film device Image intensifier Appendix H: Chapter 40 in Carlton: Fluoroscopy Slides 1-5: Fluoroscopic equipment and recording devices 6-13: Component and characteristics of the Image Intensifier (II) 14-18: Multified, FOV, and spatial resolution calculation 19: ABC 20-25: The video camera 26: Monitors 27: Coupling the II and the camera 28. Transfer of energy 29-30. Interlaced scanning 31. Digitizing the video signal 32. Fluoro gray scale Appendix H: Chapter 40 in Carlton: Fluoroscopy Cones allow photopic vision (daylight) Rods allow scotopic vision (low light) Spot films recorded on a spot film device 4 on 1 * Both were taken during fluoroscopy * Both are radiographic exposures * Both are 9 x 9 inch films made especially for this purpose, (though some fluoroscopes use standard size cassettes) 1 on 1 * Both were filmed using the same device, but in different formats. Spot film camera (Photospot) (105 mm in this case) Cine (35 mm) camera Video camera Spot film device Image intensifier 35 mm cine film. Real time motion, projected on a projector. Spot films (Photospots) recorded on a spot film (photospot) camera 105 mm identifiable by sprockets * Both were taken during fluoroscopy that drives the roll of film that is unique for this size * Both are fluoroscopic exposures (i.e. taken off the output phosphor of the II.) 105 mm * Both are serial films (not designed to be projected as a moving image, but taken in rapid sequence such as 1, 2, or 4 a second * Both are filmed by the same camera but are different sizes. 90 mm (cut film) The transfer of energy through the fluoroscopic imaging chain: The II The three major components of the fluoroscopic imaging chain are the: 1. image intensifier, 2. the video camera, and 3. the CRT monitor Image Intensifier (II) Output Phosphor Zinc Cadmium Sulfide Anode 25,000 V Electrostatic focusing lens Glass envelope Photocathode Cesium & Antimony Input Phosphor CsI Concave surface so all electrons arrive at the output screen at the same time, but causes vignetting. The transfer of energy through the fluoroscopic imaging chain Focal Point 1. Remnant to light on input phosphor 2. Light to ionized electrons on photocathode 3. Electrons back to light on output phosphor Intensification is accomplished by flux gain, and minification gain. Flux Gain Anode 25kV + potential Output Phosphor (zinc-cadmium sulfide) Electrostatic focusing lens Photocathode Electrons accelerated across the tube gain kinetic energy from the attractive force of the anode (conversion efficiency). The collision at the output screen liberates that energy in the form of more light photons Minification Gain 1” diameter Output Phosphor (zinc-cadmium sulfide) The ratio of the areas of the input and output screens is expressed as the minification gain. 92 12 = 81 times Input Phosphor (CsI) 9” diameter Total Brightness Gain The product of the flux gain and the minification gain is the total brightness gain. If the flux gain were 70, and the brightness gain 81 70 x 81 = 5670 total brightness gain 5000-30,000 is the range Quantum Mottle Because the image intensifier makes the image on the output screen thousands of times brighter than the image on the input screen, much less radiation is needed. If too few photons are used, the image becomes grainy and unacceptable for diagnostic purposes. Generally speaking a better image is always obtained by using more photons, but the price is paid in patient dose. Conversion factor The intensity of illumination at the output phosphor (candela per meter squared) to the radiation intensity that produced it (mR/s). Typical conversion factors of 50 to 300 relate to the 5000-30,000 BG Brightness gain = ratio of input to output phosphors Conversion factor = ratio of radiation intensity to brightness of output. The conversion factor is the new and best characteristic for purchasing decisions Veiling Glare Scatter radiation from x-ray, electrons, and light Multifield (Duel focus) Electronic Magnification (FOV as opposed to increased OID magnification) By increasing the positive charge on the electrostatic focusing lens, the convergence (focal) point is changed (further from out put screen). } }} 11” mode Field of View (FOV) 7” 9” 11” 7” mode FOV Duel focus or Electronic Magnification (FOV as opposed to increased OID magnification) When the convergence point is further from the output screen, (blue) the photoelectrons have further to diverge, and the image arriving at the output screen is larger. But patient dose is increased. Electronic magnification creates better resolution } }} 11” mode 7” 9” 11” 7” mode Resolution (Resolving Power) Measured in line pairs. One line and one space is a line pair 1 mm =1 line pair per millimeter of spatial resolution A Line Pair Test Tool (for Testing Spatial Resolution) The Test Tool provides line pairs of various sizes to measure spatial resolution More lines pairs per mm that are resolved = better spatial resolution Because? More line pairs demonstrate smaller things. Back to: FOV Magnification vs OID Magnification Calculating spatial resolution in lp/mm When OID is increased the image size is magnified but so is penumbra and detail is sacrificed. When FOV is decreased the image size is magnified but the spatial resolution actually improves. Here’s why: In this simplified example* let’s say we have 100 pixels across the width of the monitor. If we display 25 mm of anatomy in those 100 pixels, 4 pixels will be used to display each mm of anatomy. It takes 2 pixels to display a line pair (one pixel for the line and one for the space). Therefore, in this 25 mm FOV there is 2 lp/mm of spatial resolution. If we display 10 mm of the same anatomy in a smaller FOV, 10 pixels will be used to display each mm of anatomy, and the image will be magnified. Therefore, in a 10 mm FOV there will be 5 lp/mm of spatial resolution. Ergo, a smaller FOV = more lp/mm = better spatial resolution. Using a smaller FOV to display better spatial resolution applies to all imaging, not just fluoroscopic. So why isn’t it used exclusively? Because you don’t see as much of the big picture in a small FOV. * This simplified example does not take into account the actual number of pixels across the monitor, such as 525, 1024, 2048, or so on, and it does not include the calculation to convent inches of the FOV to mm before doing the calculation. Automatic Brightness Control (ABC) While doing fluoroscopy the .5 to 5 mA, and the kVp will be automatically adjusted to compensate for changes in part thickness and composition of the part as the fluoro tube is being moved. ABC adjusts the mA. Lag is evident, especially if the tube is moving fast. ABC also increases patient dose when using electronic magnification Vidicon and Plumbicon camera tubes The target: about the size of a dime Vidicon Camera Component parts Tube Window Signal Plate Target Anode Electron gun (cathode) Globules Control grid Steering and deflecting coils Vidicon and plumbicon cameras are analog tubes that have been in use since the early days of television. They are being replaced by CCDs. (discussed in the presentation on digital radiography) The transfer of energy through the fluoroscopic imaging chain How the camera works: The concept: The image from the output phosphor of the II must be broken down into individual units (like pixels, only called globules on the camera target). Each of these globules must encode the intensity of light by turning it into an electrical impulse that will flow out of the camera (through the pins at the rear) and send it to the monitor for display. Each globule of the camera target has a corresponding pixel on the monitor. That is, they are both in the same column and row of their respective matrices. Optical lens to focus light from II The transfer of energy through the fluoroscopic imaging chain Encoding is accomplished by the unique properties of the globule material. When excited by light, atoms from the globules raise into higher orbital shells, which makes them conductors of electricity. More intense light leads to more excitation and better conduction. In this manner the light image from the II is represented (encoded) as a matrix of globules in various stages of excited ionization (like the space charge around the filament of the x-ray tube) Optical lens to focus light from II The transfer of energy through the fluoroscopic imaging chain Then each globule must be discharged, to send the image data it contains out of the camera as electrical impulses. A stream of electrons from the electron gun showers the globules, left to right, top to bottom, (like reading a book). The steering and deflecting coils provide electromagnetic force fields that control the motion of the pencil point size foused beam. As it scans the globules on the target each globule conducts electrons through it, in proportion to the how excited (and how conductive) each particular globule is. Optical lens to focus light from II The transfer of energy through the fluoroscopic imaging chain Cable for video signal As each globule is discharged individually, the current flowing though them travels to the signal plate which is wired to pins on the rear of the tube. The series of impulses (some strong, some weak, and everything in between) flowing from the camera comprises the video signal. This encoded signal will control the intensity of fluorescence on the pixels of the monitor, and recreate the light image that came from the output phosphor of the II. The changes in the intensity of the video signal is known as modulation Optical lens to focus light from II The Monitor To recreate the image on the monitor the number of electrons stripped from the cathode (electron gun) of the tube must be controlled, for each pixel fluoresces at a different intensity. This is where the video signal comes in. The control grid on the camera supplies a constant negative potential to strip electrons from the space charge of the cathode. But the control grid on the monitor is supplied by the video signal. Since the video signals modulates as the encoded image signal, the charge it supplies the control grid is constantly changing, and a greater of lesser number of electron is fired toward the phosphor with each impulse. In this manner the each pixel of the monitor is caused to fluoresce in direct proportion to the intensity of light that was incident on its corresponding globule on the camera. The transfer of energy through the fluoroscopic imaging chain No signal High intensity Low intensity Optical lens to focus light from II Coupling of the II to the camera In the previous discussion a lens coupling was used to focus the light on the camera target. Fiber optics are a newer alternative. * Beam splitting mirror allows photospot and camera filming at the same time In conventional TV systems (not high definition) the electron beam of the monitor does not write to every line of the pixels, but to every other. First to odd numbered lines, then the screen is blanked, then to even numbered lines. Each is called a field, and there are 60 fields written a second. The combination of 2 fields is a frame and although a full frame never appears on the screen, it happens too fast for us to perceive it. At 30 frames a second we see no flicker. Interlaced Scanning Line 1 Line 2 Line 524 Line 525 262 1/2 Odd Lines scanned first = Field 1 262 1/2 Even Lines scanned first = Field 2 2 Fields = 1 Frame Question: Is the interlaced scheme desirable? No Then why is it used? It is a remnant of the original technology. What is better than interlaced? See the next presentation on High Definition Question: How is a conventional fluoroscopic, analog imaging chain converted to digital? Answer: Measure the impulses from the camera with an analog to digital converter (ADC) and store the results in a computer. If the monitor is analog a digital to analog converter (DAC) must restore the analog signal as it came from the camera. ADC 1 0 1 1 ALU CU Primary Memory (RAM) Secondary Memory DAC Finally: Why are densities reversed on the fluoro monitor? Comparison of maxillary sinuses * Low atomic densities * Low attenuation * Input phosphor glows brightly * Camera target highly excited. * Video signal is strong * Many electrons are fired from the electron gun of the monitor * Pixels glow brightly * Light area on monitor On Film On Fluoro The End